A team of Johns Hopkins University scientists discovered that noninvasive brain stimulation temporarily improves motor symptoms in patients with Parkinson's disease (PD). The Johns Hopkins study also revealed that people with Parkinson's disease are still capable of making quick, forceful movements even though most of their movements are slower and less intense than usual.

The scientists reported evidence that the patient's slowdown likely arises from the brain’s “cost/benefit analysis,” which gets skewed by the loss of dopamine in people with PD.

To test the hypothesis, experiments measured how much force a patient’s brain was willing to “assign” to each arm. In their first tests, participants, all right-handed, included 15 healthy volunteers and 15 with PD. With their arms held in mobile supports above a table, participants were asked to grip the handles at the ends of the supports, which could measure the force applied to them. Participants then had to apply about 4 pounds of muscle force to the handles to move an electronic cursor on a computer screen to a target. They could use any combination of both arms to achieve the task.

When performing the task, healthy participants shared the effort between both arms: They split the 4 pounds of force between their two arms fairly equally and never applied more than 30 percent more force with one arm than with the other. By contrast, on average, patients with PD showed a twofold greater preference for their less affected arm, sometimes skewing their effort by as much as 70 percent toward the less affected arm.

According to the team, the difference was not due to a lack of strength in the affected arm of the patients with PD, because the team also tested each arm's ability to apply force in every direction and found that the patients’ strength was comparable to that of healthy individuals.

Guessing that the reduced ability to control force in patients with PD was related to decreased dopamine — which makes it harder to “recruit” neurons for a particular task — the researchers devised a brain stimulation experiment to further test their hypothesis.

"The greater the number of neurons firing together to complete a task, the less they each have to fire and the more controlled the resulting action is,” said Reza Shadmehr, Ph.D., professor of biomedical engineering at the Johns Hopkins University School of Medicine. “In Parkinson’s, the loss of dopamine might mean that neurons that control movement don’t fire as easily, which means that a few neurons have to do the whole job and can’t perform as well, generating noisier output. The brain seems to know this and avoids assigning effort in those directions where it has less control.”

To overcome the problem, the team used transcranial direct current stimulation (tDCS) on a total of 25 patients, 10 for each of three tests. They reasoned that by increasing the electrical current within the neurons using mild stimulation through electrodes placed on the scalp, the cells would be closer to their firing threshold and would be easier for the brain to engage.

The researchers' study demonstrated that stimulation of the cortex of the brain using external electrodes corrected some of the distortion and temporarily improved some patients’ motor symptoms. Patients who received noninvasive brain stimulation split the force applied by their arms more evenly and improved motor symptoms in some patients.